Article pubs.acs.org/EF
Toward Selective Lignin Liquefaction: Synergistic Effect of Heteroand Homogeneous Catalysis in Sub- and Supercritical Fluids Rudresh Rajappagowda,*,†,‡ Abu M. Numan-Al-Mobin,† Bin Yao,† Robert Daniel Cook,§ and Alevtina Smirnova*,†,‡ †
Materials Engineering and Science Program, South Dakota School of Mines and Technology, Rapid City, South Dakota 57701, United States ‡ South Dakota School of Mines and Technology, Rapid City, South Dakota 57701, United States § Terra Forms, 8930 Ridge Trail, Sturgis, South Dakota 57785, United States ABSTRACT: Selective liquefaction of lignin is important for synthesis of value-added phenolic monomers contributing to green chemistry and sustainable energy applications. In the present study, a synergistic effect of a supercritical carbon dioxide (scCO2) acidic catalyst in combination with a heterogeneous metal oxide catalyst, specifically nickel oxide (NiO) or ceria-doped scandiastabilized zirconia (CeScSZ), in sub- or supercritical water (sbcr/scH2O) for selective liquefaction of alkali lignin is demonstrated for the first time. The scCO2-assisted hydrothermal process in the temperature range of 100−400 °C resulted in highly selective synthesis of the phenolic monomers. On the basis of the total organic carbon (TOC) and gas chromatography−mass spectroscopy (GC−MS) analysis, it is evident that the scCO2 catalyst is essential for enhancing the reaction selectivity in the presence of a heterogeneous catalyst. A combination of homogeneous scCO2 and heterogeneous NiO catalysts resulted in the highest total relative yield of the phenolic monomers, reaching ∼97% at 200 °C. It is confirmed that the scCO2-assisted hydrothermal depolymerization of the alkali lignin in the presence of both scCO2 and a heterogeneous catalyst is superior to that of the each of these catalysts alone and in comparison of the systems where scCO2 is replaced by inert nitrogen.
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Ni, Co−Mo, Ni−Mo, Al 2 O 3 , SiO 2 , SiO 2 −Al 2 O 3 , and zeolites,2,10,12,13 at 150−300 °C resulted in a 50−60% yield of aromatic monomers. Among metal-oxide-supported catalysts14 tested at 290 psi H2 and 260 °C for 8 h, 5Ru/γ-Al2O3 and 5Ru/TiO2 demonstrated up to 90% lignin conversion. At 300 °C and 20 h reaction time in the presence of Ru/γ-Al2O3 and H2, the lignin conversion reached ∼98%, indicating a substantial increase in hydrogenation and deoxygenation. The same reaction performed in N2 decreased the conversion to ∼90%, indicating that hydrogen gas is required, despite the well-documented efficiency of alcohols as hydrogen donors in other catalytic systems.15,16 However, the use of flammable gases, relatively high temperatures, and long reaction times are the drawbacks of the proposed technique. Furthermore, the chemical stability of the heterogeneous catalysts has not been addressed. In lignin liquefaction, the temperature is one of the major factors that defines the chemical composition of the final products. Specifically, at >350 °C, the MoS2 catalyst for lignin hydrogenation produces mostly aromatic hydrocarbons, which alter the products of lignin conversion and disable the competing process of repolymerization.17 Besides metals, metal alloys, metal oxides, and metal sulfides, zeolites18 also show high catalytic activity for aromatic monomer formation at 250 °C and 30−120 min reaction time, resulting in 40−60% total organic yield. However, many metal-doped and undoped zeolites produce C6−C8 aromatic hydrocarbons rather than
INTRODUCTION Lignin, as one of the most abundant byproducts derived from biomass, is currently used for power generation through either direct or indirect combustion processes.1,2 However, new and more efficient technologies, such as the recently proposed hydrothermal liquefaction of lignin in the presence of supercritical carbon dioxide (scCO2),3,4 can lead to a much higher overall commercial value. Various homo- and heterogeneous catalysts for lignin depolymerization,5−7 e.g., simple acids and bases, noble/nonnoble metals, and noble/non-noble metal oxides, sulfides, and phosphides, were used over time. Other research groups indicate that noble metals, e.g., Pt or Ru, stabilize the transition states, making them accessible for catalytic reaction of lignin decomposition without violating the principle of orbital symmetry.8 In the presence of noble metals, e.g., Pd/C catalyst, the yield of the organic liquid product from lignin is increased up to ∼80%, with a non-converted solid residue being only 1%.9 However, transition metal oxides are also competitive in comparison to the noble metal catalysts. For example, nickelbased catalysts demonstrated a 50% lignin conversion and 97% selectivity toward monomeric phenols, such as propyl guaiacol and propyl syringol, at 200 °C.10 Under milder conditions (170 °C) in water, bimetallic NiAu-based catalysts demonstrated 14 wt % lignin hydrogenolysis into aromatic monomers11 and Ni/ Mo catalysts resulted in ∼63% organic liquid product yield.9 The importance of the two major parameters, temperature and reaction time, for the systems containing various heterogeneous catalysts was emphasized earlier with respect to the aromatic yield. Conversion of lignin in the presence of carbon-supported or metal oxide catalysts, such as Pt, Ru, Pd, © 2016 American Chemical Society
Received: August 23, 2016 Revised: November 9, 2016 Published: November 10, 2016 578
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Figure 1. Schematic diagram of the experimental setup for scCO2-assisted hydrothermal liquefaction of lignin in the presence of a heterogeneous catalyst.
phenolics,2,19 in which the yield (11−43%) depends upon zeolite cavity size and decreases with the temperature. Furthermore, the inherent disadvantage of zeolites is the high yield of C1−C3 gaseous products as a result of benzene, toluene, ethylbenzene, and xylenes (BTEX) formation via ethylene-like intermediates.20 On the contrary, carbon-supported niobium oxide with coexisting Brønsted and Lewis acid sites6 used for acid-catalyzed lignin depolymerization at 260 °C in H2 produced mostly phenol and benzene intermediates and then cyclohexane because of phenol hydrogenation. An increase in the concentration of niobic acid and the Ni/Nb/C catalyst6 caused a decline in the conversion rate of the diphenyl ether lignin model compound. A comparison of the β-O-4′ bond degradation of dimeric lignin model compounds and lignin for vanadium-based catalysts demonstrated similarity in the cleavage of β-O-aryloxy bonds.21 Various combinations of heterogeneous catalysts or heteroand homogeneous catalytic mixtures were used to control the process of lignin decomposition. A synergistic effect of the two heterogeneous catalysts, specifically acidic zeolite and Raney nickel,22 was observed, resulting in ∼60 wt % organic phase and 12.9 wt % monomeric phenols. The temperature increase from 210 to 270 °C resulted in the increased yield of the phenolics from 13.6 to 25.2 wt %. A CO2/acetone/water supercritical fluid was used for depolymerization of lignin at 300−370 °C and 10 MPa. However, at higher temperatures, instead of aromatic monomers, gas, tar, or char was mostly produced.23 Despite numerous publications focused on homo- and heterogeneous catalysts, the studies of highly selective catalytic systems for synthesis of “green” polymers or economically viable synthesis of high-value organic products remain in their initial stages.7 Thus, the goal of this study was focused on a synergistic combination of homo- and heterogeneous catalysts for lignin decomposition and selective synthesis of phenolic compounds. Specifically, a scCO2 homogeneous catalyst considered as a “green solvent” was used in combination with one of the two heterogeneous catalysts, nickel oxide (NiO) and ceria-doped scandia-stabilized zirconia (CeScSZ), possessing different chemical and physical properties. The emphasis is
made on the alkali lignin decomposition in a sub- or supercritical water (sbcr/scH2O)−scCO2 environment at different temperatures. To confirm the advantages of the sbcr/scH2O−scCO2 system, the results are compared to the corresponding data obtained in the sbcr/scH2O−supercritical nitrogen (scN2) system. Performed for the first time, this study is a starting point for the economically viable selective synthesis of high-value organic products from lignin with a potential to decrease the harmful environmental effects of carbon dioxide and minimize the amount of lignin waste in a sustainable way.
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EXPERIMENTAL SECTION
Materials. Alkali lignin and 4-chloroacetophenone from SigmaAldrich were used for this study. Deionized water was obtained using a Milli-Q Integral Water Purification System (EMD Millipore Corp., Billerica, MA, U.S.A.). For liquid−liquid extraction, acetic acid and dichloromethane (DCM) of gas chromatography (GC) quality were obtained from Sigma-Aldrich (Atlanta, GA, U.S.A.). The reactor, tubing, and fittings were purchased from the High Pressure Equipment Company (Erie, PA, U.S.A.) and Swagelok (Solon, Ohio, U.S.A.). NiO was purchased from Wako Pure Chemical Industries, Ltd. (Lot WAG 3361), and CeScSZ was synthesized by a solid-state approach24 with a composition of 1 mol % CeO2, 10 mol % Sc2O3, and 89 mol % ZrO2. Hydrothermal Treatment of Lignin in sbcr/scH2O−scCO2/ sbcr/scH2O−scN2. The treatment of the alkali lignin at sub- and supercritical conditions in a temperature range of 200−400 °C in the presence of heterogeneous NiO and CeScSZ catalysts was carried out in a stainless-steel 12 mL high-pressure reactor (316 SS) with a pressure tolerance of up to 103 MPa (Figure 1). A type K thermocouple was inserted through the bottom of the reactor to measure the inside temperature by an AMProbe temperature meter. The pressure sensor connected to the monitor controlled the internal pressure. To achieve reproducible synthesis conditions, the temperature ramp was adjusted in each experiment. To perform an experiment, 0.1 g of lignin and 6 mL of deionized water were deposited inside the reactor. The NiO or CeScSZ heterogeneous catalysts in the amount of 0.1 g were added to the reactor that was placed inside the split Carbolite furnace for scCO2 hydrothermal treatment. To reach the targeted temperature within the shortest period of time, the initial setup temperature of the Carbolite temperature controller was adjusted to 650 °C with a ramp rate of 100 °C/min. Dependent upon the synthesis conditions, after 2−5 min, the temperature was adjusted to the required value. A 260D Teledyne 579
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Figure 2. Characterization of the lignin before scCO2-assisted hydrothermal treatment: (a) TGA, (b) FTIR, and (c) NMR data. linkage in guaiacyl aromatic methoxyl groups, and the peak at 1244 cm−1 confirms the presence of the syringyl ring and C−O stretching. The peaks that appeared in the range of 1260−1270 and 1330−1375 cm−1 exhibit the guaiacyl and syringyl groups in the lignin. NMR spectra was taken by a Bruker Advance 500 MHz high-field superconducting NMR spectrometer (with multinuclear solution capability) (Figure 2c). Liquid Sample Analysis. The organic phase collected after the scCO2 hydrothermal treatment was separated from the aqueous phase by liquid−liquid extraction (LLE) using DCM. In the first step of extraction, 10 μL of acetic acid was added to every 1.0 mL of a liquid sample, resulting in the pH value of ∼2.0. Then, the recovery standard (4-chloroacetophenone) was added, enabling us to monitor and correct for losses during the extraction. The sample was then extracted 3 times with 1.0 mL of DCM with vigorous shaking. After separation of the DCM bottom layer and the aqueous top layer phases, the three bottom layers were combined and the internal standard was added. The relative yield of each compound obtained for GC−MS analysis was calculated as a percent ratio between the area under the GC−MS peak of this compound divided by the total area of all of the GC−MS peaks. The GC−MS analysis was performed using GC-MS-QP2010 Ultra injections (1.0 μL) in a split mode using linear velocity of 32.4 cm/s with deactivated glass wool (Restek, Bellefonte, PA, U.S.A.). A Rxi-5sil MS column, 0.25 μm thick and 0.25 μm in diameter, was used for all separations. Ultrapure helium (99.999%) was used as a carrier gas with a constant flow rate of 1.0 mL/min. The initial oven temperature was set up to 45 °C for 1.0 min, ramped to 280 °C with a rate of 10 °C/ min, then ramped to 320 °C with a rate of 20 °C, and held for 5 min. The injector and transfer line temperatures were set up at 250 and 280 °C, respectively. All of the MS data were acquired in a total ion current (TIC) mode with a mass range of m/z 45−500 and a scan speed of 2500. The weight of the solid phase before and after hydrothermal reforming in scCO2 was measured gravimetrically. At 300 and 200 °C, the lignin conversion was ∼50 wt % and 30 % wt % of the lignin, respectively. In terms of the total yield and mass balance, only solid
syringe pump pressurized with CO2 or N2 maintained a constant pressure of 22.063 MPa in the reactor. When the pressure and the temperature requirements were met, a stopwatch was used to record the reaction time. During this time, the temperature and pressure inside the reactor were continuously monitored. After 10 min, the furnace was turned off and the pressure was released. The reactor was quenched by immersion in cold water. Lignin Characterization. The lignin from Sigma-Aldrich is characterized by pyrolysis−gas chromatography−mass spectroscopy (pyr−GC−MS), thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), and nuclear magnetic resonance (NMR) (Figure 2). The results of pyr−GC−MS of the untreated alkali lignin (starting lignin) and solid-state phase produced in H2O−CO2 at two temperatures, 250 and 350 °C, are presented in our earlier publication.3 These data show that the starting lignin has the same compounds as the lignin hydrotreated at 250 and 350 °C. Also, it was confirmed that phenolic products make up the solid phase formed as a result of lignin depolymerization; therefore, the solid-phase product can be used as an additional source of valuable phenolics.3 TGA was performed using a SDT Q600 (TA Instruments) in the temperature range from 25 to 800 °C at a heating rate of 10 °C/min in air. In each experiment, 0.1 g of the sample was analyzed. According to TGA (Figure 2a), the decomposition of the original lignin starts at 350 °C and almost 80% of lignin is decomposed at 480 °C. FTIR (Cary 660 FTIR spectrometer) from Agilent Technologies was used to analyze the lignin sample (Figure 2b). A strong absorption around 3500 cm−1 was found and assigned to the −OH stretching vibration of aliphatic and aromatic groups in the lignin. The absorption peak at 2900 cm−1 belongs to the stretching vibration of the C−H band of CH2, CH3, and CH3O groups in lignin. The peak assigned to CO stretching is at 1740 cm−1. The peak in the fingerprint (1650 cm−1) is for O−H and conjugated C−O bonds. The peak values at 1598 and 1510 cm−1 are usually contributed from aromatic skeletal vibration (CC) in lignin. The peaks found at 1465 and 1426 cm−1 are caused by the stretching in the phenol−ether bonds of the lignin. The peaks at 1268 cm−1 can be assigned to C−O stretching and C−O 580
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In both types of fluid mixtures, the temperature increase to 300 °C causes an increase in the TOC concentration for both NiO and CeScSZ catalysts. However, the most significant TOC yield at 300 °C is observed for CeScSZ in the presence of scCO2, reaching ∼100 mg/L. At 400 °C, the TOC values are higher in scH2O−scN2, indicating that scCO2 does not have an advantage over N2, which could be explained by the lower solubility of scCO2 in scH2O, as emphasized in our previous publication.3 Total Phenolic Yield. The GC−MS analysis of the organic phase produced after the scCO2-assisted hydrothermal liquefaction (Figure 4) demonstrates that, for most temperatures and both heterogeneous catalysts, the presence of scCO2 is beneficial with regard to the relative yields of the phenolic products.
and liquid phases were taken into consideration. The mass of the gases, e.g., CH4, CO, CO2, or H2, that are usually produced at the temperatures above the operation temperature range of 250−350 °C25 was considered negligible in comparison to the weight of the solid and liquid products. Evaluation of the total concentration of the organic carbon produced in the process of lignin liquefaction was performed by the total organic carbon (TOC) analysis using the Shimadzu TOC-L CPH/CPN carbon analyzer with an eight-port OCT-L auto sampler. For the TOC analysis, the liquid aqueous−organic phases from the reactor after lignin treatment at 200, 300, and 400 °C were filtered and centrifuged at 12 500 rpm for 10 min to remove the solid phase. The centrifuged liquid was diluted with distilled water to 33% by volume. Distilled water was used as a TOC blank standard. All of the experiments were repeated for reproducibility, and the results were comparable to each other.
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RESULTS AND DISCUSSION TOC Analysis. The results for the total organic carbon produced from the alkali lignin in the presence of NiO or CeScSZ catalysts after scCO2 hydrothermal treatment at 200, 300, and 400 °C are compared for two fluid mixtures, specifically sbcr/scH2O−scCO2 and sbcr/scH2O−scN2 (Figure 3). On the basis of the X-ray diffraction (XRD) analysis of NiO
Figure 3. Results of the TOC analysis after the liquefaction of alkali lignin in the presence of NiO and CeScSZ catalysts in two types of fluid mixtures: sbcr/scH2O−scCO2 and sbcr/scH2O−scN2.
before and after the scCO2-assisted catalytic lignin reforming, a conclusion was made that it is stable at the processing temperatures of 200, 300, and 400 °C and 10 min reaction time. In general, NiO has low solubility in water, even in acidic solutions at 350 °C.26 Stabilized zirconia is even more stable in an acidic environment and is used for corrosion protection in thermal barrier coatings. At 200 °C, the highest amount of TOC is produced in a mixture of sbcrH2O and scCO2 in the presence of CeScSZ (42 mg/L), which is approximately 3 times higher than that for NiO in scCO2. In comparison to the sbcr/scH2O and scN2 fluid mixture, the TOC value in scCO2 at 200 °C and especially at 300 °C is higher for CeScSZ but significantly lower for NiO (10 mg/L). This TOC loss, evident at all three temperatures for the NiO−scCO2 catalytic system, could be due to the interaction of NiO with CO2.27 In this case, the formation of nickel carbonates taking place on the surface of the NiO catalyst can partially deactivate the NiO active surface sites. Furthermore, the lower TOC values in the case of the NiO− scCO2 mixture could be due to the higher catalytic activity of NiO in comparison to CeScSZ and the possible loss of carbon in the form of methane or carbon dioxide.28
Figure 4. Relative total yield of the phenolic compounds formed from the alkali lignin in a process of hydrothermal treatment in scCO2 or scN2 and in the presence of (a) NiO and (b) CeScSZ heterogeneous catalysts.
The highest total phenolic yield of ∼97% is observed in the case of NiO (Figure 4a) at 200 °C in the presence of scCO2, which is almost twice as high in comparison to the total phenolic yield obtained from lignin in the presence of nitrogen. In comparison to NiO, CeScSZ fluorite is less effective in terms of the formation of the phenolic products, resulting in the maximum phenolic yield of ∼72% observed at a higher temperature (250 °C) than that for NiO (200 °C). It is necessary to note that the effect of scCO2 in increasing the total phenolic yield is higher at lower temperatures, which is consistent with our previous results,3,4 emphasizing that scCO2 is more effective in the presence of subcritical rather than 581
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Energy & Fuels scH2O, which could be due to the decreased solubility of scCO2 in supercritical water at higher temperatures. In the case of NiO combined with scCO2 (Figure 5a), the total phenolic yield produced from the hydrothermal lignin
Figure 6. Relative yields of the specific phenolic products from the hydrothermal treatment of the alkali lignin in a mixture of sbcr/scH2O for the CeScSZ catalyst in (a) scCO2 and (b) scN2. Figure 5. Relative yields of the specific phenolic products from the hydrothermal treatment of the alkali lignin in a mixture of sub- or supercritical fluids in the presence of the NiO catalyst and (a) scCO2 and (b) scN2.
phenolic yield and selectivity, especially for guaiacol (panels a and b of Figure 6), is explicit. In comparison of scCO2 versus scN2, the highest selectivity toward guaiacol is observed for the CeScSZ−scCO2 system and reaches its maximum (∼50%) at 400 °C. In the case of CeScSZ−scN2, an opposite tendency takes place: starting at ∼21% at 200 °C, the relative yield of guaiacol decreases to ∼14% at 400 °C. Thus, less selective than NiO, the CeScSZ catalyst in combination with scCO2 is a good example of the synergistic effect of both catalysts that is responsible for the reaction selectivity and the yield of the specific phenolic compounds. A comparison of the two catalytic systems with NiO or CeScSZ heterogeneous catalysts in the presence of scCO2 compared to scN2 (Figures 5 and 6) provides a useful approach for tuning the catalytic mixtures toward more efficient selective phenolic synthesis. The selectivity of the proposed catalytic systems toward specific phenolic compounds (Figures 6 and 7) was evaluated from the GC−MS spectra in correlation with Table 1. Effect of scCO2 in the Presence and Absence of the Heterogeneous Catalyst. While discussing the selectivity enhancement for the heterogeneous catalysts in the presence of scCO2, an additional comparison of these systems to those that do not contain a heterogeneous catalyst is important. The corresponding GC−MS results for four major phenolic products, specifically vanillin, guaiacol, creosol, and vanillyl ethyl ether, produced in the presence of NiO and CeScSZ are presented in comparison to the relative yields obtained in the absence of the heterogeneous catalysts (panels a−d of Figure 7).
liquefaction is ∼97%. A combined synergistic effect of NiO and scCO2 results in high selectivity toward synthesis of vanillin (∼58%) at 200 °C. However, this selectivity is gradually decreasing, with the temperature reaching only 1% at 400 °C. On the contrary, the relative yield of guaiacol at 200 °C is ∼22% and continues to increase, with the temperature reaching the maximum relative yield of ∼60% at 350 °C. These results indicate the complexity of the processes that take place in the reaction media and necessity of careful reaction optimization to produce specific phenolic products. In comparison to the lignin liquefaction in NiO−scCO2 hydrothermal conditions, substitution of scCO2 for scN2 (Figure 5b) leads to significant loss of the total phenolic yield (∼53%), which is about half of what is obtained in the case of scCO2. Interestingly, in terms of vanillin, the decreasing trend in the yield with the temperature is similar to what is observed for scCO2-assisted hydrothermal reforming (Figure 5a) but the absolute values are smaller (∼25% at 200 °C), clearly emphasizing the efficiency of the combined effect of the homogeneous scCO2 and heterogeneous NiO catalysts. In comparison to the NiO catalyst, CeScSZ fluorite in scCO2 demonstrated less pronounced catalytic activity in terms of both the total phenolic yield and selectivity (Figure 6a). However, the effect of scCO2 fluid in increasing the total 582
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Figure 7. Relative yields of the four major phenolic compounds formed in the process of the lignin scCO2-assisted hydrothermal treatment at different temperatures and constant pressure in sbcr/scH2O−scCO2 or sbcr/scH2O−N2 fluid mixtures and in the presence of NiO or CeScSZ: (a) vanillin, (b) guaiacol, (c) creosol, and (d) vanillyl ethyl ether.
depicted in Figure 8. In terms of lignin depolymerization, the cleavage of these β-O-4 bonds is considered as one of the most important reaction mechanisms.29 Being the predominant linkage in the lignin structure,30 the cleavage of the β-arylether bond depends upon the rate-determining step, which is the removal of the β proton from the lignin monomers. This, in turn, depends upon the type of conjugate base employed for the process.29 On the basis of the experimental results, it can be concluded that scCO2 significantly enhances the catalytic effect of the heterogeneous catalyst with regard to the β-O-4 bond cleavage. In this case, scCO2 plays a dual role. Specifically, scCO2 improves the overall catalytic activity of the system with a heterogeneous catalyst. At the same time, scCO2 minimizes the mass-transport limitations within the lignin porous structure, thus enhancing depolymerization of the lignin surface. It is possible to assume that, interacting with sbcr/scH2O, scCO2 provides a source of protons that are known to facilitate β-O-4 bond cleavage. On the other hand, maintaining CO2 in its supercritical state, scCO2 facilitates cleavage of the aryl-ether bonds as a result of the high solubility of the forming monomers in an acidic fluid mixture of scCO2 and sbcr/ scH2O.3,4 Moreover, high diffusivity of scCO2 allows for deep penetration within the lignin pores,31 which is not possible in the case of common aqueous solutions or aqueous−organic mixtures. In addition to the advantages mentioned above, scCO2 prevents the formation of oligomers, leading to repolymerization, which significantly enhances the selectivity of the liquefaction process.32 All of these benefits enable scCO2
In the case of vanillin (Figure 7a), all of the systems with or without a catalyst in scCO2 or scN2 demonstrate the same trend of decreasing the yield with the temperature. The highest yield of vanillin (∼58%) was obtained at the lowest temperature (200 °C) in the NiO−scCO2 system, emphasizing the advantage of the NiO heterogeneous catalyst in combination with scCO2 over the inert nitrogen atmosphere. On the contrary to vanillin, guaiacol (Figure 7b) has an opposite trend in terms of the relative yield versus temperature. Guaiacol starts to form at lower temperatures, but its yield reaches the highest value of ∼60% at 350 °C in sbcrH2O− scCO2−NiO. However, synergistic enhancement of the heterogeneous catalyst by scCO2 is also visible in this case and results in an increased relative yield from ∼38% for sbcrH2O−scCO2 to ∼45% for the sbcrH2O−scCO2−NiO system. In comparison to vanillin and guaiacol, the relative yield of creosol (Figure 7c) is much smaller. It gradually increases with the temperature reaching the highest value of ∼16% at 400 °C for CeScSZ in scCO2. However, it also confirms the synergistic effect of scCO2 that improves the total phenolic yield and increases the selectively toward creosol. The fourth major compound, vanillyl ethyl ether (Figure 7d), produced along with guaiacol, vanillin, and creosol, demonstrated a higher yield in the temperature range of 200−300 °C, with the highest value of ∼25% in the presence of NiO−scCO2 at 300 °C. Reaction Mechanism in the Presence of scCO2. The nature of the bonds within the lignin polymer molecule and the position of the β-O-4 bond subjected to cleavage in the presence of scCO2−sbcrH2O and a heterogeneous catalyst is 583
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Energy & Fuels Table 1. Tentatively Identified Lignin-Related Derivatives from GC−MS (NIST11) Library
a
name of the compounda
IUPAC name
RTb
m/z
quantification ion
o-cresol p-cresol benzene, 1,2-dimethoxyphenol, 3,5-dimethylphenol, 3-ethylcreosol phenol, 3-ethoxym-guaiacol 2,3-dimethoxytoluene 1,2-benzenediol, 3 methylphenol, 4-ethyl-2 methoxyphenol, 4-(aminomethyl)-2-methoxy2-ethoxy-4-methylphenol benzene, 4-ethyl-1,2 dimethoxyphenol, 2,6-dimethoxyeugenol 4-ethylcatechol vanillin trans-isoeugenol phenol, 2-butylphenol, 2-methoxy-4 propyl1,3-benzenediol, 4-propyl1,2-Dimethoxy-4-n-propylbenzene 2-propanone, 1-(4-hydroxy-3-methoxyphenyl)benzene, 4-butyl-1,2-dimethoxymethyl-(2-hydoxy-3-ethoxy-benzyl)ether
2-methylphenol 4-methylphenol 1,2-dimethoxybenzene 3,5-dimethylphenol 3-ethylphenol 2-methoxy-4-methylphenol 3-ethylphenol 2-methoxyphenol 1,2-dimethoxy-3-methylbenzene 3-methylbenzene-1,2-diol 4-ethyl-2-methoxyphenol 4-(aminomethyl)-2-methoxyphenol 2-ethoxy-4-methylphenol 4-ethyl-1,2-dimethoxybenzene 2,6-dimethoxyphenol 2-methoxy-4-prop-2-enylphenol 4-ethylbenzene-1,2-diol 4-hydroxy-3-methoxy benzaldehyde 2-methoxy-4-[(1E)-1-propen-1-yl] phenol 2-butylphenol 2-methoxy-4-propylphenol 4-propylbenzene-1,3-diol 1,2-dimethoxy-4 propylbenzene 1-(4-hydroxy-3-methoxyphenyl)propan-2-one 4-butyl-1,2-dimethoxybenzene 2-ethoxy-6-(methoxymethyl) phenol
8.26 8.65 9.71 9.77 10.04 10.28 10.42 10.92 11.11 11.40 11.49 11.75 11.81 12.30 12.71 12.82 13.12 13.43 13.52 14.02 14.16 14.35 14.78 15.08 15.28 16.51
108 108 138 122 122 138 138 124 152 124 152 153 152 166 154 164 138 152 164 150 166 152 180 180 194 180
C7H8O C7H8O C8H10O2 C8H10O C8H10O C8H10O2 C8H10O2 C7H8O2 C9H12O2 C7H8O2 C9H12O2 C8H11NO2 C9H12O2 C10H14O2 C8H10O3 C10H12O2 C8H10O2 C8H8O3 C10H12O2 C10H14O C10H14O2 C9H12O2 C11H16O2 C10H12O3 C12H18O2 C10H12O3
The names of compounds are based on the identification using the MS NIST library. bRT = retention time.
heterogeneous catalysts specifically (NiO and CeScSZ) is demonstrated. The synergistic effect of scCO2 and heterogeneous catalyst NiO along with H2O in its subcritical state results in a total relative yield of phenolic compounds up to ∼97% and selectivity for vanillin up to ∼58% at 200 °C. Although the heterogeneous catalysts were reported to be more effective than their homogeneous counterparts, it is noteworthy to mention that no single catalyst can be efficient enough for the selective production of a specific phenolic compound or a homologous group of specific phenolic compounds. It has been confirmed for the first time that a combination of a homogeneous carbon dioxide catalyst in its supercritical state with a heterogeneous metal oxide catalyst is beneficial for the decrease of the operation temperature and selectivity enhancement toward the formation of the specific phenolic compounds. However, the proposed concept of tuning the scCO2-assisted hydrothermal system with respect to a particular phenolic compound by carefully varying the key reaction parameters,34 such as the temperature, pressure, volume of the reactant, reaction time, lignin loading, and catalytic ratio, requires further consideration. The knowledge of all of these parameters will allow us to scale up the scCO2-assisted hydrothermal system to an industrial level. The proposed new catalytic system designs will meet the demand for various materials in the organic polymer industry and can potentially minimize the environmental impact of both lignin and carbon dioxide waste. Considering that the process of lignin reforming is exothermic, it is a challenge to monitor and measure the experimental parameters in short intervals of time. Future research will be focused on the design of the continuous flow scCO2-assisted hydrothermal systems for lignin liquefaction, which allows us to monitor the liquefaction process at each
Figure 8. Viable cleavage of β-O-4 bonds in lignin, leading to selective formation of phenols in the process of lignin hydrothermal liquefaction in the presence of scCO2 and a heterogeneous catalyst.
to be an excellent tunable “green” solvent, also performing a role of a homogeneous catalyst. It has been reported10 that the β-aryl-ether bond in lignin can be selectively cleaved using a carbon-supported nickel catalyst in an ethanol−benzene mixture. As final products, propylguaiacol and propylsyringol with total selectivity of >90% at 50% lignin conversion (12 h at 200 °C and 500 rpm) were produced.10 It is assumed that a similar mechanism can take place in this study, although enhanced by the presence of scCO2. It is expectable that β-O-4 bonds as the bonds with the smallest dissociation energy will cleave first.33 As a result, scCO2 in combination with a heterogeneous catalyst provides a tunable approach for selective synthesis of high-value phenolic products.
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CONCLUSION The highly selective scCO2-assisted hydrothermal reforming of lignin into specific phenolic compounds at relatively low temperatures (200−350 °C) in the presence of two 584
DOI: 10.1021/acs.energyfuels.6b02128 Energy Fuels 2017, 31, 578−586
Article
Energy & Fuels
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discrete temperature and pressure and tune the system for less energy consumption and a more selective, efficient, and continuous scCO2-assisted hydrothermal process, leading to selective production of industrially important organic compounds.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. edu. *E-mail:
[email protected]. ORCID
Rudresh Rajappagowda: 0000-0002-0854-1675 Alevtina Smirnova: 0000-0003-1520-0331 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This study is supported by the National Science Foundation Experimental Program to Stimulate Competitive Research (EPSCoR) Track II “Dakota BioCon” Project for North and South Dakota (Grants IIA-1330840 and IIA-1330842).
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DOI: 10.1021/acs.energyfuels.6b02128 Energy Fuels 2017, 31, 578−586
